Occurrence of red spotted masu salmon in the Jinzu River inhabited by masu salmon

摘要

Red spotted masu salmon, which is called amago or satsuki masu, Oncorhynchus masou ishikawae, caught mixed together in the catch with masu salmon, O. masou masou, in the Jinzu River, one of main fishing rivers of masu salmon in Japan, was investigated from 1991 to 1995. The body size (range 21.5-44.0 cm) of red spotted masu salmon caught in the Jinzu River was markedly smaller than thai (range 43.5-70.0 cm) of masu salmon. This means that the period and area of ocean migration of red spotted masu salmon was shorter and smaller, respectively, than that of masu salmon. A red spotted masu salmon caught in the Jinzu River weighed 3.2 kg (FL 62.0 cm), which equaled the mean body weight of masu salmon. This suggests the possibility of cross mating occurring between red spotted masu salmon and masu salmon. The rale of red spotted masu salmon caught with masu salmon in the coatal waters near the mouth of the Jinzu River ranged from 6.9 percent to 14.6 percent during five years. The fork length of red spotted rnasu salmon caught in the sea ranged significantly less (16.0-46.0 cm) than that (13.0-72.0 cm) of masu salmon. It is considered that red spotted masu salmon should be excluded from the Jinzu River, and we should defend the intrusion of juveniles of red spotted masu salmon in order to keep masu salmon body size large and propagate masu salmon resources effectively in the Jinzu River.